An Experimental Evaluation of Density Dependence in a Subtidal Algal Population

• Published in: Ecology
• Main Author: Reed, Daniel C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1990
• Subjects: Ecology, Evolution, Behavior and Systematics; White Sea
• Online Access: http://dx.doi.org/10.2307/1938639; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.2307%2F1938639; https://esajournals.onlinelibrary.wiley.com/doi/pdf/10.2307/1938639

The effect of density on the survival, growth, fecundity, and recruitment of the subtidal kelp Pterygophora californica was experimentally evaluated by thinning a natural population of 1—yr—old plants to four different densities. Tagged individuals were followed through time, and changes in their estimated tissue volume were calculated from in situ morphometric measurements. Highly seasonal patterns of growth and reproduction were observed. During fall senescence, plants in stands thinned to the lowest density (i.e., 2 plants/m 2 ) showed a significantly greater decline in volume than those in denser stands (i.e., 58 and 102 plants/m 2 ). During periods of high growth, lower density plants showed significantly greater gains in volume and were the only plants that became reproductively mature. In the winter when volume was lowest, the proportion of individuals in the smallest size classes decreased as treatment density increased. In contrast, in the summer when volume was greatest the predominance of small individuals increased as treatment density increased. New recruits of Pterygophora as well as other macroalgae were observed only in lower density stands. Lower density plants also had higher levels of epiphytes. The distributions of the white sea urchin (Lytechinus anemesus) and two sea hares (Aplysia californica and A. vaccaria), the three most conspicuous grazers during this study, were correlated with Pterygophora density. Lytechinus were significantly more abundant in low—density plots, while Aplysia spp. were common only in high—density plots. Both grazers appeared to remove substantial amounts of biomass, and there was no obvious effect of stand density on the amount of plant tissue lost as a consequence of grazing. These findings suggest that density can strongly influence patterns of growth and reproduction in this alga. A likely mechanism is competition for light at high density. The relative importance of light competition at higher densities, however, probably varies with other factors such as ...